The following is part of series of guest posts from Windows IT Pro that takes an in-depth look at SMTP.

Last week we shared the first in a series of posts on the history of Simple Mail Transfer Protocol (SMTP) from our friends at Windows IT Pro. The first post explained the origins of SMTP and how it’s influenced how we email today. It ended by saying:

The use of IP means that we can depend on SMTP to eventually get the message contents to a server, but what happens to it after that is up to the server. Imagine what happens when a delivery driver leaves a package at your office on a Saturday. What happens to it after delivery isn’t his problem. Deliverability, monitoring, tracking, authentication, and encryption are all examples of services that SMTP doesn’t necessarily provide itself but that are still very valuable.

We talk a lot about email deliverability best practices and troubleshooting here on the blog. But it’s always good to get another perspective and reminder on the ins and outs of deliverability. The following will take a look at what it means to get delivered.

What Does “Deliverability” Mean?

Simply put, deliverability is an indication of whether your messages will get to their intended destinations. For actions such as sending order status notifications, shipping or tracking updates, or sending time-sensitive announcements, deliverability is critical because messages that don’t get to the customer aren’t useful to them or to you.

When Is a Message Delivered?

You can think about message delivery in several ways. Consider the case in which Alice, a user at Contoso, is sending a message to Bob, a customer who works at Fabrikam. When is her message delivered? Is it when:

  • Her message leaves her client and is accepted by a Contoso server? After all, at that point her client is done with the message, similar to what happens when you drop a letter in the outbound mailbox at an office building.
  • The message leaves the Contoso network and is passed to a Fabrikam server? This is similar to sending a package to a company with a centralized mailroom—just because someone at the target company has signed for it doesn’t mean that the intended recipient got it.
  • Her message is placed in Bob’s mailbox on the target server?
  • Bob actually opens and reads the message?

There’s no single correct answer because the meaning of “delivered” varies depending on the context of the message. In most cases, though, we can consider a message delivered once it arrives in the target mailbox. (Though technically “delivered” is when you receive a “250 OK” reply at the end of an SMTP transaction, as “250 OK” is indicative of successful actions throughout the SMTP Protocol.)

In contrast to paper mail, where most of us would think a letter is delivered once it arrives at the recipient’s mailbox, some argue that email deliverability focuses more on whether the recipient reads the message and not just on whether it reaches its destination. This can be determined by tracking whether a recipient opens or clicks on the message.

Temporary and Permanent Delivery

It’s important to understand that deliverability problems fall into two categories: temporary and permanent. Temporary problems, commonly known as “soft bounces,” might delay the delivery of a message, but eventually the message will get through (typically SMTP servers will automatically retry soft bounces for up to 72 hours ). Examples include transient network problems, connection throttling, or a recipient’s mailbox being full; SMTP MTAs provide the ability to queue and retry messages for these problems.

Permanent problems (or “hard bounces”), such as a nonexistent recipient address, won’t be resolved with time. It’s important to monitor these two bounce types and update your mailing lists accordingly. If you implement an email service provider, like SendGrid, they can provide detailed data about your recipients’ behavior back to you, so you can remove unresponsive subscribers from list. To learn how SendGrid handles bounces, check out our blog post on email bounce management.

Stay tuned for the next post in this Windows IT Pro series that dives into factors that affect deliverability. And if you can’t wait, you can always check out our Deliverability Guide and our guide on The ABCs of ISPs.


This post is courtesy of our friends at Windows IT Pro.



Jillian Wohlfarth
As SendGrid's Director of Content, Jillian is responsible for ensuring that SendGrid provides valuable thought leadership content through the blog, whitepapers, webcasts, and more. When not writing and editing, you can find Jillian frequenting Denver restaurants in search of the best queso.